A worldwide interoperability for microwave access (WiMAX) system has a variety of frame lengths of 2, 5, 10, and 20 ms. When a minimum frame size is assigned for a measurement gap to discover a preamble of a frame start position, the opportunity to discover the preamble is given at least once. However, in the WiMAX system having a frame size of 20 ms, if a mobile station of a long term evolution (LTE) system successively consumes 20 ms or more in order to discover an adjacent WiMAX base station, the mobile station may encounter a serious service delay. Accordingly, a method of reducing the size of one measurement gap and assigning the measurement gap at regular patterns or randomly has been proposed.
FIG. 1 is a view illustrating a process for discovering a preamble of an adjacent WiMAX base station according to a measurement gap size while a mobile station of an LTE system provides an evolved universal terrestrial radio access network (E-UTRAN) service.
Referring to FIG. 1, {circle around (a)} shows a case where one measurement gap size is 5 ms and half of one radio frame of 10 ms is assigned for a measurement gap. The measurement gap of 5 ms is randomly assigned within one radio frame and WiMAX preambles are detected in the second and fourth measurement gaps. However, since the third and fourth measurement gaps are successively assigned, the mobile station can not receive an E-UTRAN service for a total of 10 ms. Therefore, as shown in {circle around (b)} of FIG. 1, one measurement gap size may be reduced to 2.5 ms and a measurement gap may be randomly assigned within one radio frame. However, since a time assigned for the measurement gap is less than the case of {circle around (a)}, a time for discovering the WiMAX preamble may be delayed.
When handover is performed in the LTE system, it is necessary to determine a channel measurement interval to recognize the existence of an adjacent base station and to report a channel state. However, in the case where a candidate base station differs from a currently connected base station in frequency or radio access technology (RAT), there has been no way to allocate an interval for discovering adjacent base stations and measuring a channel state of corresponding base stations while minimizing a service interruption time with the currently connected base station.